Sieve Analysis and Determination of Grain Size Distribution of Coal for Identifying Ideal Sieving Time When Performing Coal Beneficiation Using Gravity Separation

The beneficiation of coal through gravity separation plays a key role in industry, allowing for particle size distribution determination and separation of coal in to various sizes for future combustion in boilers, and separation of impurities. For a complete combustion an industrial suspension boiler using bituminous coal requires an average particle size of 45μm and 80 to 85% of particles to be less than 200mesh. This size requirement varies depending on the type and size of boiler and the rank of coal being used; a fixed bed (stoker) combustion method requires an average particle size of 0.25in, a fluidized bed requires and average particle size of 0.04in, a suspension combustion method requires an average particle size of 45μm. The purpose of this experiment was to determine the ideal amount of time for a sieve shaker to run, in order to achieve a more accurate reading of particle distribution for combustion in a common industrial suspension boiler or determine if another type of boiler may be ideal, based on size distribution. This size distribution was found using sieve analysis, passing a known quantity of coal through sieves of decreasing size using various durations (5, 10, 15, and 20 min) in a sieve shaker; then measuring the quantity of coal left in each sieve and calculating the percentage of coal that fell between that sieve size. For each time period, little variability between particle distributions below the average grain diameter of 0.1in was found, leading to the conclusion that 5 min of sieve shaking was necessary for extraction of particles for usage in suspension boilers for power generation. This was only roughly 2±0.1% of the coal sample, leading to the conclusion that more comminution would be required to achieve a larger usable amount. Roughly 38±0.8% of the coal sample would be suitable in a fluidized bed boiler, which suggested that this level of comminution was suitable for fluidized combustion. A fixed bed boiler required an average size of 0.25in, which was outside of the range of average size measured by the sieves used. However, it is safe to assume that less than 20±2% would be usable in fixed bed boiler. Introduction The beneficiation of coal promotes homogeneity in the size of pulverized coal after comminution, as well as helping with the separation of impurities that result in ash and sulfur production when coal is burned. (1-2) One method of beneficiation is gravity separation via the screening of coal particles through sieves of decreasing size. (1) Depending on boiler type and combustion method, a different particle size of coal may be required for efficient combustion; a fixed bed (stoker) combustion method requires an average particle size of 0.25in, a fluidized bed requires and average particle size of 0.04in, a suspension combustion method requires an average particle size of 45μm with typically less than 2% by weight greater than 50 mesh (300μm), 60 to 70% less than 200mesh (74μm) for lignites and subbituminous coals, and 80 to 85% less than 200mesh for bituminous coals. (3) An expected pyritic sulfur reduction after beneficiation of a ICO coal can range from 22 to 72%, an ICO coal also has been known to show a reduction in ash yield of 78% after beneficiation. (1) In industry, the separation of coal sizes and resulting homogeneity of size at each separation after beneficiation allows for the coal to be effectively and efficiently utilized, while also reducing ash and sulfur output upon combustion. (1-2)